Mechanical gearboxes are extensively used to match the operating speed of prime-movers to the requirements of their loads for both increasing the rotational speed such as, for example, in a wind-powered generator or reducing rotational speed such as, for example, in an electric-ship propulsion arrangement. It is usually more cost and weight effective to employ a high-speed electrical machine in conjunction with a mechanical gearbox to achieve requisite speed and torque characteristics. However, while such a high-speed electrical machine in conjunction with a mechanical gearbox allows high system torque densities to be realized, such mechanical gearboxes usually require lubrication and cooling. Furthermore, reliability can also be a significant issue. Consequently, direct drive electrical machines are employed in applications where a mechanical gearbox cannot be used.
There are various direct drive electrical machine topologies. One such topology is the permanent magnet rotary/linear homopolar (transverse-field) machine (TFM) that is known to offer the highest torque/force density. A rotary TFM has a torque density in the range of 40-60 kNm/m3. However, homopolar machines have inherently poor power factors that are of the order of 0.3-0.45, which makes them unsuitable for electrical power generation. Furthermore, they require a significantly higher converter volt-ampere rating for motor applications.